1. Field of the Invention
The present invention relates to a nitride semiconductor light emitting diode (LED) which maintains low ohmic contact resistance, thereby enhancing the light extraction efficiency.
2. Description of the Related Art
In general, a nitride semiconductor is such a material that has a relatively high energy band gap (in the case of GaN semiconductor, about 3.4 eV), and is positively adopted in an optical device for generating green or blue short-wavelength light. As such a nitride semiconductor, a material having a composition of AlxInyGa1-x-y)N (herein, 0≦x≦1, 0≦y≦1, and 0≦x+y≦1) is widely used.
However, since such a nitride semiconductor has a relatively large energy band-gap, it is difficult to form the ohmic contact with an electrode. Particularly, since a p-type and n-type nitride semiconductor layers have a larger energy band-gap, the contact resistance on the contact portion with p-type and n-type electrodes increases. Such an increase causes an operational voltage of the device to increase, thereby increasing the heating value. Further, the p-type and n-type electrodes of a conventional nitride semiconductor LED are formed of Cr/Au with low reflectivity. Therefore, light emitted from an active layer is not totally reflected, but is partially absorbed, thereby reducing the light extraction efficiency.
Such a nitride semiconductor LED is divided into a lateral LED and a vertical LED.
Now, the conventional nitride semiconductor LED will be described in detail with reference to drawings.
First, a lateral nitride semiconductor LED of the conventional nitride semiconductor LED will be described with reference to FIG. 1.
FIG. 1 is a sectional view illustrating the structure of the conventional lateral nitride semiconductor LED.
As shown in FIG. 1, the conventional vertical nitride semiconductor LED 100 includes a sapphire substrate 110, a GaN buffer layer (not shown), an n-type nitride semiconductor layer 120, an active layer 130, and a p-type nitride semiconductor layer 140, which are sequentially grown. Portions of the p-type nitride semiconductor layer 140 and the GaN/InGaN active layer 130 are removed by mesa-etching, so that a portion of the upper surface of the n-type nitride semiconductor layer 120 is exposed.
On the exposed n-type nitride semiconductor layer 120, a negative electrode (n-electrode) 170 is formed of Cr/Au. On the p-type nitride semiconductor layer 140, a positive electrode (p-electrode) 160 is formed of Cr/Au.
The n-type nitride semiconductor layer 120 and the p-type nitride semiconductor layer 140 have a large energy band-gap. Therefore, if the n-type nitride semiconductor layer 120 and the p-type nitride semiconductor layer 140 respectively come in contact with the n-electrode 170 and the p-electrode 160, the contact resistance increases. Such an increase causes an operational voltage of the device to increase, thereby increasing the heating value.
The p-electrode 160 and the n-electrode 170 are formed of the same material, such as Cr/Au, and simultaneously come in ohmic contact with the p-type and n-type nitride semiconductor layers 140 and 120, thereby simplifying the process. However, the p-electrode and n-electrode absorbs some light emitted from the active layer, thereby reducing the entire luminous efficiency of the device. That is, since the p-electrode and n-electrode formed of Cr/Au have low reflectivity, light directed to the p-electrode and n-electrode among light emitted from the active layer is absorbed. Therefore, the light extraction efficiency decreases, thereby reducing the brightness of the device.
Referring to FIG. 2, the conventional vertical nitride semiconductor LED will be described.
FIG. 2 is a sectional view illustrating the structure of the conventional vertical nitride semiconductor LED.
As shown in FIG. 2, the conventional vertical nitride semiconductor LED 200 includes an n-electrode 170, an n-type nitride semiconductor layer 120, an active layer 130, a p-type nitride semiconductor layer 140, a p-electrode 160, and a support layer 210, which are sequentially formed.
In the vertical nitride semiconductor LED, however, the n-type nitride semiconductor layer 120 and the p-type nitride semiconductor layer 140 have a large energy band-gap. Therefore, when the n-type nitride semiconductor layer 120 and the p-type nitride semiconductor layer 140 respectively come in contact with the n-electrode 170 and the p-electrode 160, the contact resistance increases. Such an increase causes an operational voltage of the device to increase, thereby increasing the heating value.
Further, since the p-electrode 160 and the n-electrode 170 are formed of Cr/Au, some light emitted from the active is absorbed, thereby reducing the entire luminous efficiency of the device.